A typical wall-flow honeycomb filter includes a monolith having longitudinal, generally parallel cells (or channels) defined by porous walls. The cells are alternately end-plugged to form a checkered pattern of plugs at the inlet and outlet end faces of the monolith. The cells having their ends plugged at the inlet end face of the monolith and open at the outlet end face of the monolith are referred to as outlet cells, and the cells having their ends plugged at an outlet end face of the monolith and open at an inlet end face of the monolith are referred to as inlet cells. In a standard cell configuration, the ratio of open cells to plugged cells at either of the end faces of the monolith is typically 1. The inlet and outlet cells typically have a square shape, perhaps because square cells are easier to manufacture and lend themselves to a regular pattern of alternating inlet and outlet cells having equal cross-sectional areas for low pressure drop and backpressure. In operation, exhaust gas or other particulate-laden flow enters the wall-flow honeycomb filter through the inlet cells, is forced from the inlet cells into the outlet cells through adjoining porous walls, and exits the filter through the outlet cells, with the porous walls retaining a portion of the particulates in the flow.
The filter may be catalyzed to reduce pollutants such as hydrocarbons and CO from the flow prior to the flow exiting the filter. The filter may be catalyzed by coating the porous walls with a washcoat containing active catalytic species and/or depositing the washcoat within the pores of the porous walls. In comparison to a bare filter, the effective flow area of the inlet cells may decrease as the thickness of the washcoat on the porous walls increases. A decrease in effective flow area could result in an increase in pressure drop across the honeycomb filter and a corresponding increase in system backpressure. If the washcoat is deposited within the pores of the porous walls, the effective flow area of the inlet cells may be largely unaffected. However, as the catalyst loading within the pores increases, the storage capacity of the filter may decrease because the pores that would otherwise be available for collecting particulates may now be filled partially or entirely with the washcoat.